This invention relates to boring apparatus useful, e.g., in earth or rock.
Typically such apparatus has a frame supported at the lower end of a drill pipe string, with a pilot bit mounted at the bottom of the frame and reamers for enlarging the pilot hole mounted on the frame above the pilot bit.
A number of considerations enter into the building of such boring apparatus. The apparatus should be strong, be capable of withstanding high torque and of boring fast through hard rock, and should not break apart in the bore hole; on the other hand the apparatus should be of limited diameter to fit the hole produced and should be inexpensive, have few parts, be simple, not require undue maintenance and have long life. Difficulties arise in discovering designs to balance these somewhat conflicting considerations to improve the efficiency of use of the capital invested.
Of the various prior known designs, two having some similarity with the present invention will be mentioned.
In one reamer-stabilizer design, having rollers for engaging the side walls of the bore hole, it was suggested that the rollers be mounted permanently, enabling the exterior frame surfaces above and below the rollers to be formed as smooth surfaces of revolution, avoiding discontinuities that constitute wear points in the harsh hole bottom environment where the frame bears continuously against the jagged rock walls. See U.S. Pat. No. 3,306,379.
Drawbacks of the design are that it does not take a large cut and must be discarded when any of the bearings, shafts or cutters fail, despite useful life remaining in the main frame and other parts.
In another design the rollers are arranged in conical configuration on axes that converge downwardly. Each roller has integral stub-shafts at both ends, the upper stub shaft held in a discrete bearing block bolted to the frame and the lower stub shaft held in a large bearing inserted into the frame. In the steps of assembly, each roller is placed in position with its lower stub shaft protruding downwardly into a hole in the frame, the lower bearing is slipped upwardly from below through a passage that has been pre-drilled from the frame bottom, and the upper bearing block is assembled upon the upper stub shaft and bolted in place. Thereafter a threaded connector for the pilot bit is bolted to the frame.
This design has the advantage over the previous one mentioned of taking a larger cut. But the various exposed bolts and pieces suffer such damage during rotation against the rock that the rollers cannot thereafter be removed, hence the useful life of the entire assembly is again limited to that of the roller cutters.